The present invention relates to adsorption heat exchangers.
Heat exchangers for exchanging heat between gaseous and liquid media are used in a wide range of large-scale and high-value industrial applications, including: air drying and dehydration in open and closed systems, water removal from volatile organic compounds (VOCs) or natural gas, solvent recovery, gas separation (e.g. nitrogen from air), water separation from ethanol (in biofuel production), or CO2 and N2 separation from natural gas. Among further heating and cooling applications are adsorption-based heat pumps with an adsorption heat exchanger as the central part, which may, for instance, be used for energy efficient data centers with internal heat recovery.
An efficient technology for providing the physical contact which is necessary for exchanging heat with the gaseous medium is adsorption, the attachment of atoms or molecules from the gaseous phase on external surfaces of the heat exchanger. These surfaces are made from materials with a high heat conductivity to provide a good thermal coupling between the two fluids interacting in the heat exchanger.
The challenge of designing an effective adsorption-based heat exchanger is to find the best compromise between maximizing its thermal coupling performance by offering a large surface for adsorbing the gas while keeping low resistances for both the mass transport and the heat transport and ensuring a maximum of mechanical robustness. Key figures for characterizing an adsorption heat exchanger are the cooling power per unit volume, or volumetric sorption power (VSP), i.e. the amount of heat transferred in an evaporator per liter of internal volume of the adsorption-based heat exchanger, and the volumetric efficiency, which is the percentage of the internal volume which is not used by heat transport channels or mass transport channels.
Known from the state of the art are fixed-bed adsorbers, in which an adhesive may be used to attach individual adsorbent beads to each other and to the surface of a heat exchanger. However, there is an inherent trade-off between improving mass transfer (smaller particles) and improving thermal transport (reduce number of interfaces) which limits the sorption rate.
A more effective technology uses adsorbent coatings on lamellae to improve the thermal contact between adsorbent and heat exchanger by enlarging the contact area. The coating is either formulated using a binder or direct synthesis of the adsorbent on the heat exchanger. However, mass transport within the coating is poor, which limits the maximum coating thickness, leading to a poor volume utilization. A VSP of 140 watts per liter (W/L) was reported for this technology in “Experimental investigation of the effect of zeolite coating thickness on the performance of a novel zeolite-water adsorption heat pump module”, Proceedings of the 10th International Conference Enhanced Building Operations (ICEBO), Kuwait, October 2010, by B. Dawoud et al.
Yet higher adsorption efficiencies have been achieved by the use of adsorbent coatings on metal fibers, a technology still under development. Coated fibers feature improved thermal transport by applying the adsorbent coating on a network of metallic fibers while the vapor transport is improved by the empty percolating channel space between the fibers. A VSP between 200 and 500 W/L was reported for this technology in “Performance evaluation and optimization of adsorption modules”, presented at Sorption Friends Meeting, Milazzo, Sicily, September 2015, by G. Füldner.